The invention relates to a method of solderless aluminum brazing and a flux suitable therefor.
In brazing, metal workpieces conventionally are joined by means of a molten solder. Fluxes are used in brazing to provide for a clean surface and thus for a strong joint between the materials. Fluxes have already been developed which perform this task of aluminum brazing very well. Fluxes based on potassium fluoroaluminates have proven especially suitable. The preparation of potassium fluoroaluminates and the use thereof as fluxes is described for example in U.S. Pat. Nos. 3,951,328, 4,428,920, 4,579,605, and 5,318,764, published German patent application Nos. DE 195 19 515 (= U.S. Ser. No. 08/590,636), DE 195 20 812 (= U.S. Ser. No. 08/658,179), and DE 195 37 216 (= U.S. Ser. No. 08/726,432). U.S. Pat. No. 4,906,307 discloses fluxes for brazing aluminum-based alloys wherein the flux contains 70 to 90 wt-% of K.sub.2 SiF.sub.6 and 30 to 10 wt-% of AlF.sub.3. This flux can be used, if desired, with other additives such as lithium fluoride or sodium fluoride. A dispersant can also be used.
In the aluminum brazing methods disclosed in the prior art, in addition to the flux, a metal solder must always be applied to the metal components to be joined. According to U.S. Pat. Nos. 5,100,048 and 5,190,596, this can be accomplished by adding a metal, such as silicon, copper or germanium to the flux. According to other methods, aluminum members are used which have been plated with the solder metal. A disadvantage of these known methods is that they always include a process step which calls for the application or the admixture of the solder metal or other metals such as silicon, copper or germanium.